Supplying in constant level form a reservoir or tank containing a random liquid, which is liable to flow out or evaporate, makes it necessary to carry out various operations involving the detection of the level, the interpretation of the signal obtained and then, on the basis of this, acting on the flow rate at which the liquid enters the tank.
When the liquid is helium, whose boiling temperature, which is a function of the vapor pressure, can be in the range 1 to 5 degrees absolute, one of the essential conditions to be satisfied is to limit to the maximum heat passing onto the bath to be checked.
The means generally used for measuring the helium level are either capacitive or superconductive probes giving a continuous measurement, or resistance point probes giving an "all or nothing" signal.
The generally electrical signal is collected and processed by electronic means operating at ambient temperature, where they actuate the electrical or pneumatic control of a regulating valve, whose cold active element (seat and needle) controls the necessary supply flow rate.
In the special case where the helium is in the superfluid state, at temperatures below T.lambda.=2.17.degree. K., very special properties appear and these can be utilized for permitting either the detection of a liquid - gas interface, or the checking of its position. These very curious properties are linked with the existence of a helium isomer, namely helium II, which only appears below T.lambda. and which more particularly gives rise to the absence of a triple point of the helium, whose vapor tension and fusion curves do not meet.
Among the usable properties which only appear in the superfluid liquid phase of helium are the Fontaine pressure effect and the so-called Rollin film phenomenon. Brief details are given of these below in order to facilitate the understanding of the device according to the invention.